Minimap or UI indicators

[thombruce]

#25, #27, #28 <- These together are going to result in the existence of objects that may now be off screen as the player moves. With no other reference frames, it will be hard to find the planets/stars again.

Consider either:

1. A minimap
2. On-screen indicators pointing towards off-screen objects in the system

At this point, happy to go with whichever is easier and do some crude implementation (maybe draw a shape at the edge of the screen in the direction of each object - maybe also add the relative distance to it).

This will be reworked later, but is for now crucial to include as part of #27 before v0.0.10

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In a future update, consider using https://www.kenney.nl/assets/simple-space for a minimap element. Also consider use of a UI camera that draws different sprites than the main one - not sure how this works, but it should be possible and useful for both a minimap and a system map (openable on maybe the M key).

[thombruce]

Useful example in the Bevy docs: https://bevyengine.org/examples/2D%20Rendering/rotation/

[thombruce]

Idea: Hidden planets. Planets that do not show up in the indicator system (until you're practically on top of them).

There could even be entire systems like this - uncharted - implying that the systems where planets are indicated are all registered in a ship-board database.

Indicators would become visible at a distance determined by the planets' sizes (whether or not they would be visible and discernible as planets if the game weren't top down).

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Another thought: Too many indicators. There is no reason that all of the moons (which could be many) of a planet should be indicated when you are nowhere in the vicinity of said planet - just indicate the planet and later point to the moons when in the planet's SOI.

What about important space stations in orbit around planets?

Indicators could feature icons which describe their children, be they amenities like space stations or a number of moons.

We could also implement priorities that override exclusion of indicators.